This invention relates to a process for the whitening of synthetic gypsum obtained by reacting phosphate rock with sulfuric acid. In particular, it relates to the treatment of synthetic gypsum with ozone, and to the products produced thereby.
The commercial production of phosphoric acid by reacting sulfuric acid with phosphate rocks results in the formation of by-product gypsum of the formula Ca SO.sub.4.2H.sub.2 O. The so-obtained gypsum, generally called phosphogypsum, even after purification by washing with water, retains deleterious color-imparting impurities. Therefore, it is not possible to use phosphogypsum in certain fields of application, for example as a filler in the paper industry.
Whereas, after washing with water, phosphogypsum has a whiteness grade in the range of 63 to 70 dependent upon the particular nature of the treated phosphate rock, it is necessary in the paper industry for bulk fillers to have a whiteness grade (measured according to French Standard NF Q 03-038), higher than 75 and coating fillers to have a whiteness degree higher than 86.
To obtain white phosphogypsum, attempts were made to treat phosphogypsum by bleaching agents commonly used, for instance, to treat cellulosic fibers. Unfortunately, treatment of phosphogypsum with bleaching agents such as sodium hypochlorite, calcium hypochlorite, chlorine dioxide, hydrogen peroxide, sodium bisulfite cannot be satisfactorily conducted under industrial conditions. Moreover, difficult handling and pollution problems are associated with the use of these bleaching agents.